Apparatus for and method of producing metal

ABSTRACT

A method involving the production of metal, in particular, aluminum by providing an electrolytic bath containing dissolved oxide of the metal to be produced in a reduction cell. A direct current flows through the bath and metal is collected on the bottom of the reduction cell. The method includes determining the undesirable process generated noise component of the resistance in a reduction cell. The method preferably involves reducing or eliminating the noise component whenever it exceeds a given level. The apparatus for producing metal includes at least one reduction cell having electrodes for delivering direct current to an electrolytic bath containing dissolved oxide of the metal. A circuit arrangement is operatively arranged to sense the process generated noise component of the resistance on the reduction cell and produces an output signal whenever this component exceeds a given level indicating the existence of a noise level which is detrimental to efficient cell operation. The apparatus preferably includes devices responsive to the output from the circuit arrangement for reducing or eliminating the noise component whenever it exceeds the given level.

United States Patent [1 1 Murphy APPARATUS FOR AND METHOD OF PRODUCINGMETAL [75] Inventor: Joseph A. Murphy, Murraysville,

[73] Assignee: Southwire Company and National Steel Corporation,Pittsburgh, Pa.

[22] Filed: Feb. 21, I973 [21] Appl. No.: 334,233

Related U.S. Application Data [63] Continuation-impart of Ser. No.298,405, Oct. 18,

[52] U.S. Cl 204/64 R; 204/67; 204/225; 204/228; 204/245 [51] Int. Cl.C22d 3/00; C22d 3/02; C22d 3/12 [58] Field of Search 204/67, 243 R-247,204/228, 64 R, 225

[ 1 Apr. 15, 1975 Primary Examiner-John H. Mack Assistant Examiner-D. R.Valentine Attorney, Agent, or Firm-Van C. Wilks; Herbert M. Hanegan;Stanley L. Tate [57] ABSTRACT A method involving the production ofmetal. in particular, aluminum by providing an electrolytic bathcontaining dissolved oxide of the metal to be produced in a reductioncell. A direct current flows through the bath and metal is collected onthe bottom of the re duction cell. The method includes determining theundesirable process generated noise component of the resistance in areduction cell. The method preferably involves reducing or eliminatingthe noise component whenever it exceeds a given level. The apparatus forproducing metal includes at least one reduction cell having electrodesfor delivering direct current to an electrolytic bath containingdissolved oxide of the metal. A circuit arrangement is operativelyarranged to sense the process generated noise component of theresistance on the reduction cell and produces an output signal wheneverthis component exceeds a given level indicating the existence of a noiselevel which is detrimental to efficient cell operation. The apparatuspreferably includes devices responsive to the output from the circuitarrangement for reducing or eliminating the noise component whenever itexceeds the given level.

24 Claims, 2 Drawing Figures APPARATUS FOR AND METHOD OF PRODUCING METALREFERENCE TO RELATED APPLICATTON This application is acontinuation-in-part application of the application of Joseph A. Murphy,entitled A METHOD OF AND APPARATUS FOR PRODUC- lNG METAL, Ser. No.298,405 which was filed on Oct. 18, 1972.

BACKGROUND OF THE INVENTION This invention relates to a method of and toan apparatus for the control of an electrolytic reduction cell or cellsfor producing molten metal. The invention relates, more particularly, toa method of and to an apparatus for the control of an electrolyticreduction cell or cells in which a metallic compound or soluteconstituent of a fused electrolyte in an electrolytic cell produces amolten metal. The invention is directed, in its primary adaptation, tothe control of an electrolytic cell or cells useful in the production ofaluminum.

The production of aluminum by electrolysis of an aluminum containingcompound is a very old, wellknown process. Commercial aluminumproduction is carried out by the Hall-Heroult process in which aluminumoxide, refined from bauxite ore, is reduced electrolytically. Alumina,A1 the solute, is dissolved in molten cryolite, NaFlAlF the solvent, ata temperature of about 970 C. The dissolved alumina, when subjected to ahigh intensity current, in electrolytic cells of either the continuous,self-baking Soderberg anode type or the pre-baked anode type,disassociates into positive aluminum and negative oxygen ions. Inpractice, a plurality of substantially identical electrolytic reductioncells, for example 28 reduction cells, are arranged in a pot line, thatis, they are connected electrically in series. A direct current of fromabout 50,000 amperes to I60,000 amperes or more, in commercial reductionsystems, is usual. The electrical path for the external current sourceis composed of the carbon anode structure, the electrolytic bath and thecathode structure, usually in the form of collector bars buried in thebottom of the reduction cell. The specific current, in any case beingdetermined by the siize of the electrolytic reduction cells, flowsthrough the bath containing the alumina and electrolyte, a voltage dropof from about four volts to about six volts appearing across eachreduction cell during normal volts to about six volts appearing acrosseach reduction cell during normal electrolysis. As the normalelectrolysis proceeds, aluminum is deposited at the cathodic bottom ofthe reduction cell or each of the series-connected reduction cells whereit collects as a molten pool of aluminum, a tap being provided in eachof the reduction cells so that the aluminum can be periodically removed.The side of the reduction cell which is provided with the tap forremoving the molten aluminum is known as the tap side. The oxygen of thealumina combines with the carbon of the anode to form principally carbondioxide and carbon monoxide, the gases being conventionally led awayfrom each reduction cell by a duct, the duct being positioned near thetop of that side of each cell which is opposite to the tap side. Thisparticular side of a reduction cell is referred to generally as the ductside.

According to Faradays Law, the pounds of aluminum produced are directlyproportional to the quantity of electrical charge passed through each ofthe reduction cells. An approximate equivalent circuit for an individualreduction cell would show a decomposition voltage of back EMF in serieswith a resistance having a fixed component and variable component. Thefixed component is determined by the electrical resistance of mechanicalcircuit connections, while the variable component represents theresistance of the bath itself. The bath resistance, in turn, can beexpressed as R,. p (D/A), where A, the effective surface area, isessentially constant, but P, the resistivity, varies with the aluminaconcentration, and D, the anode to cathode spacing, varies with anodeconsumption as well as with aluminum buildup on the cathodic bottom ofthe reduction cell. The thermal input to the reduction cell, and hencetemperature of the cell, depends on the R losses generated in thisresistance.

The efficiency of the process, at least in terms of pounds perampere-hour is determined by the percentage of metallic aluminum which,under the influence of strong magnetic fields, comes into contact withoxygen near the anode to reform the original oxide. The tendency forthis to happen increases with temperature and decreases with thedistance between the anode and the cathode. Although aluminum productiondepends on ampere-hours, users of electrical power actually pay on akilowatt-hour basis; consequently, efficiency in terms of cost dependson maximizing current in the volt-ampere relationship. For efficientoperation, the undesirable process generated noise component of theresistance of the reduction cell should be reduced or eliminated,special precautions being taken to assure that this noise component ismaintained below a given tolerable level. The resistance of thereduction cell should preferably be regulated to provide additionally alow stable bath temperature with high current and minimum reoxidation.

One of the continuing problems encountered in the commercialelectrolytic aluminum reduction process is the effective control of theconcentration of dissolved alumina in the bath. If the concentration ofalumina is depleted from the upper maximum of from about 7% to about 10%down to a certain critical limit, generally considered to beapproximately 2.0%, a phenomena known as anode effect occurs, with itsconsequent wellknown disadvantages and reduced efficiency. The anodeeffect is a charactetistic of reduction cells in which aluminum is beingproduced by electrolysis of a cryolite/alumina bath. The anode effect isconventionally extinguished and normal electrolysis restored by theexpedient of breaking the frozen top crust of the bath which addsalumina into the bath. Extreme caution must be taken, however, not tocharge the bath with too much additional alumina for all of theadditional alumina will not dissolve if the amount exceeds a solubilitycapacity of the electrolyte for alumina at the prevailing temperature,usually about 970 C. If the electrolyte cannot dissolve all of theadditionally added alumina, some of the alumina will sink through theelectrolyte and through the molten alumina, collecting on the cathodicbottom surface of the reduction cell, with the result that theresistance of the cathode undesirably increases, efficiency declinesresulting in what is known as an over-fed or sick reduction cell.

In both cases the anode effect, which results from a starvationcondition of the bath, and the sick cell phenomena which results fromoverfeeding the bath, the

reduction cell is working under abnormal conditions with the concomitantundesirable decline in overall efficiency. Of the two conditions, theanode effect has been found to be the lesser of the two disadvantagesfor it can be extinguished more easily than the sick cell condition canbe remedied. Consequently, techniques have been developed, involvingboth intermittent and continuous alumina feeding of an electrolyticbath, which add alumina to the electrolytic bath routinely in amountsadapted to avoid development of a sick cell condition. Such feedingtechniques rely on an underfeeding practice, which allows the reductioncell to undergo occasional anode effects, for example, one anode effectper day, which assures against overfeeding alumina into the reductioncell.

The US. Pat. to Bruno, et al, No. 3,400,062, issued Sept. 3rd, 1968,discloses a control system for an aluminum reduction cell having ananode, a cathode and a fused electrolytic bath of cryolite and dissolvedalumina. A pilot anode is insulatively supported at the reduction cell,with one extending into the electrolytic bath to a given immersiondepth. A power supply is provided for supplying direct current energy tothe pilot anode. The current density supplied to the pilot anode duringa standby, which may be of several minutes duration, is sufficientlyhigh to maintain the auxiliary anode on anode effect. This particularcondition, according to the patent specification, is maintained toprevent consumption of the pilot anode by the electrolysis. During asecond period of from about 8 to about 10 seconds duration, for example,the current to the pilot anode is reversed in order to eliminate itsanode effect. Finally, during a third period of from about ID to aboutseconds a second or lower voltage, preselected in dependence on thelevel of alumina control desired, is impressed upon the pilot anodecausing the resulting current through it in the forward direction toprovide a given control current density in order to determine thealumina concentration of the electrolytic bath by sensing whether or notan anode effect appears on the pilot anode under this lower voltagecondition. After a brief pause of a few seconds to allow the current inthe pilot anode to stabilize, a sensor associated with the pilot anodeis made effective for detecting whether or not an anode effect hasappeared. if an anode effect has appeared, it is an indication that theelectrolytic bath is in an underfed condition, and the feed rate for thealumina into the bath is increased. On the other hand, if no anodeeffect appears during this sensing period, the normal slower feed ismaintained. Thus, it is clear that the system disclosed in the priorpatent to Bruno et al is rather complex and, so far as the reversecurrent and sensing periods are concerned, requires from about 18 toabout 38 seconds to operate.

The US. Fat. to Smids, No. 3,539,456, issued Nov. 10th, I970, disclosesan electrolytic cell solute determining apparatus for use in theoperation of a direct current electrolytic reduction cell in which apair of alternating current energized auxiliary electrodes, which extendinto an aluminum oxide/solute containing bath of the electrolyticreduction cell, serves as a means for sensing the current of cyclinganode effects induced thereon during operation of the reduction cell.This particular apparatus, because the alternating current energizationof the pilot electrodes, does not require the current reversal in thepilot electrode as was needed in the direct current energized pilotelectrode arrangement disclosed in the above-mentioned patent to Bruno,et al. It is to be appreciated, however, that the anode effect sensingapparatus disclosed in the above-mentioned patent to Smids stillrequires a relatively long period to operate and requires that a specialalternating current supply and auxiliary electrodes be provided.

The U.S. Pat. to Brown, No. 3,345,273, issued Oct. 3, 1967, discloses amethod of and an apparatus for indicating the position of an anode withrespect to a cath ode in a multiple anode reduction cell. The relativeposition is determined by measuring the amplitude of a low frequency[-20 c.p.s.) voltage signal which is superimposed on the direct voltageapplied across the reduction cell during operation. These low frequencyvoltage variations, which may be designated process generated noise,occur whenever the anode or a portion thereof is too close to the anodethereby causing an overload of the anode. The faulty spacing may resultfrom a number of causes. An operator may, in approximating the properposition of a replacement anode block, incorrectly position thereplacement block. An anode or anode block, may be inadvertentlydisturbed during normal operation. Waves may be produced in the bath, ahumping or thickening of the aluminum layer under one of the carbonblocks or a portion of the anode structure may occur, a condition whichmay, at least for a period, drastically reduce the thickness of thecryolite layer resulting in an upset condition.

Faulty spacing between the anode and the cathode of a reduction cell mayresult from portions of an anode or different anode blocks beingconsumed at different rates. In some instances, pieces of the carbonanode may fall off, resulting in improper spacing. Undesirable processnoise may also result from chunks of ore either shorting the anode tothe cathode or reducing the resistance of the anode-to-cathode path inthe bath.

The above-mentioned patent to Brown discloses the concept of sensing themagnitude of the low frequency voltage signal which appears across areduction cell and using the sensed signal to actuate an alarm wheneverit exceeds a predetermined level, indicating too little spacing betweenthe anode and cathode. The sensed low frequency voltage signal may, forexample, be fed to a DC. voltmeter causing its pointer to oscillate overa range of 0.2 volts or more, indicating to an operator that the anodeis too close to the cathode. An on-line computer may be used tocalculate the amplitude from a set of consecutive voltage readings andissue preselected instructions to an operator.

SUMMARY OF THE INVENTION It is an object of the present invention toprovide a method of producing metal from an electrolytic bath whichinvolves sensing the occurrence of the undesirable process generatednoise component of the resistance in a reduction cell.

is is another object of the present invention to provide a method ofproducing metal from an electrolytic bath which involves determining theoccurrence of a process generated noise component of the resistance in areduction cell in excess of a given level, and reducing the noisecomponent below the given level.

It is a further object of the present invention to provide, in anapparatus for the reduction of metal from an electrolytic bath, acircuit arrangement for the detec- *ion of the process generated noisecomponent of the .esistance in a reduction cell.

It is yet another object of the present invention to rovide, in anapparatus for the reduction of metal "rom an electrolytic bath, acircuit arrangement which :nses the occurrence of the process generatednoise component of the resistance and effects the reduction orsubstantial elimination of the detected noise component whenever itexceeds a tolerable level.

It is yet a further object of the present invention to provide, in anapparatus for the reduction of metal from an electrolytic bath, aninexpensive and reliable circuit arrangement for sensing the occurrenceof the arocess generated noise component of bath resistance.

The foregoing objects, as well as others which are to 1e made apparentfrom the text below are accomplished according to the present inventionin its method aspect by providing an electrolytic bath containingdissolved oxide of the metal to be produced in a reduction cell, causingdirect current to flow through the bath and collecting the metalproduced on the bottom of the eduction cell. The method includes sensingthe process generated noise component of resistance of the lath anddetermining when this noise component ex- :eeds a given level as anindication of an undesirable noise level. The method may also involvereducing the noise level whenever it exceeds the given level.

The foregoing objects, as well as others which are to be made apparentfrom the text below, are accomplished according to the present inventionin its apparatus aspect, in an apparatus which includes at least onereduction cell having electrode means for delivering direct current tothe bath. Circuit components are provided for sensing the processgenerated noise component of bath resistance. Additional circuitcomponents are operatively arranged to be responsive to output from thecomponents for sensing for determiniing the occurrence of noisecomponents in excess of a given level. The apparatus further may includedevices responsive to the output from the components for sensing whichreduce the noise level whenever it exceeds the given level.

BRIEF DESCRIPTION OF THE DRAWING FIGS. 1A and 1B are schematicillustrations of an apparatus for producing metal from an electrolyticbath in accordance with an illustrative apparatus embodiment of thepresent invention, the apparatus being particularly suitable forcarrying out the method according to the present invention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT As seen in FIGS. 1Aand IB, an alumina reduction cell, generally designated 9, withassociated circuitry, suitable for practicing the present invention isshown schematically. The alumina reduction cell 9 includes a steel shell10 having a carbonaceous lining 11. The conductive lining 11 contains apool of molten aluminum l2 and a bath 13 of alumina dissolved in amolten electrolyte, the bath 13 being above the pool of molten aluminum12. Conductive rods, which are embedded in the conductive lining 11, areconnected to a cathode conductor or bus 14. It is to be understood thatother forms of lining can be used to contain the molten aluminum l2 andthe bath 13. A cathode potential can be impressed on the molten aluminum12 by other conventional means instead of the conductive rods as shown.Suspended above the bath l3, and partially immersed therein, is a carbonanode 15 shown diagrammatically. In practice, the carbon anode 15 may bea multiple bar anode arrangement positioned on a suitable superstructureadjustable as a unit or a conventional vertical or horizontal stubSoderberg-type anode. One multiple bar anode arrangement which can beused for the anode 15 comprises eighteen carbon bars, each weighingabout one ton. The molten bath 13 is covered by a hard crust 16 whichconsists of frozen electrolyte constituents and additional alumina. Theanode I5 is connected to a positive bus 17 via a conductor 18. A currentsensing device 20 is provided for sensing the current flowing in theconductor 18. The current sensing device 20, which produces a directvoltage directly related to the direct current flowing in the conductor18, preferably is of a type which does not require a series connectionin the conductor 18.

On the tap side of the reduction cell 9 (Le, the side from which moltenaluminum may be drawn off), a first conventional alumina feeder 24 isprovided. A first crust breaking bar 25 is provided in the vicinity ofthe first feeder 24. A second alumina feeder 26 is provided on the ductside of the reduction cell 9 (Le, the side from which gases may be drawnoff) and a second crust breaking bar 27 is provided in the vicinity ofthe second feeder 26. Two pneumatically or electrically operatedmotion-producing devices 28 and 30 mechanically connected to the anode15 are provided respectively for raising and lowering the anode 15 inpredetermined increments. A volt meter 31 is connected between thenegative bus 14 and the conductor 18.

A pulse producing timing circuit 32 is provided for producing two pulsetrains, each having an identical pulse repetition rate, for example, apulse repetition rate of 5 pulses per minute. The two pulse trains areout of phase, one pulse train being displaced from the other by one-halfthe interval between pulses, for example, by 6 seconds. One of the pulsetrains from the timing circuit 32 is fed to the enabling input of a gatecircuit 33 and the other pulse train is fed to the enabling input of agate circuit 34. The signal input of the gate circuit 33 is connected tothe current sensing device 20 and receives therefrom a voltage signaldirectly proportional to the current flowing in the conductor 18. Thesignal input of the gate circuit 34 is connected to the conductor 18 andreceives therefrom a voltage corresponding to the voltage across thereduction cell 9.

The respective outputs from the gate circuit 33 and the gate circuit 34are coupled to the input of a limiting amplifier 35 which is preferablyoperatively arranged to limit at an input voltage of approximately 10volts. The limiting amplifier 35 preferably has a gain of one. Theoutput from the limiting amplifier 35 is coupled to an analog to digitalconverter 36 which produces binary coded digital signal outputs whichcorrespond, at different times, to the current supplied to the reductioncell 9 and to the voltage drop across the reduction cell 9, asdetermined by which of the two gate circuits 33 and 34 is supplying aninput to the limiting amplifier 35.

The output from the analog to digital converter 36 is coupled to thefirst input of an AND circuit 37 and to the first input of an ANDcircuit 38, second inputs to the AND circuit 37 and to the AND circuit38 being connected to the timing circuit 32 for receiving the respectivepulse trains therefrom. Thus, the AND circuit 37 intermittently passesto its output a binary coded digital signal indicative of the directcurrent flowing within the reduction cell 9 and the AND circuit 38intermittently passes into its output a binary coded digital signalindicative of the voltage across the reduction cell 9.

The AND circuit 38 has its output coupled to a first input of asubtractor 39. A second input to the subtractor 39 is connected to abinary coded digital signal source 29 which is settable and provides asits signal output a predetermined binary coded digital signalrepresenting the back EMF of the reduction cell 9, this back EMF beingnominally 1.6 volts for an alumina/- molten cryolite bath. The outputsignal from the sub tractor 39 is fed to a first input of anarithmetical circuit, denominated as an arithmetic divider 40. The ANDcircuit 37 has its output couplled to a second input of the divider 40via a digital store 41 which stores the binary coded digital signalreceived from the AND circuit 37 for a sufficiently long period toassure that the divider 40 has present contemporaneously at its twoinputs the signals passed by the AND circuits 37 and 38. The divider 40produces as its output a binary coded digital signal which is thequotient of the digital signal representing the gross voltage across thereduction cell being examined minus the back EMF of the cell divided bythe digital signal representing current, its binary coded digital outputsignal thus corresponds to the resistance of the reduction cell 9, itselectrodes and connections thereto.

The output from the divider 40 is coupled to a first input of anarithmetic subtractor 43 which is operatively arranged to receive at itssecond input a predetermined binary coded digital signal from a digitalsignal source 42, which signal represents the known fixed electricalresistance of the electrical connections to the reduction cell 9.Accordingly, the subtractor 43 produces as its output signal a binarycoded signal substantially directly corresponding to the varyingresistance of the bath 13.

The output from the subtractor 43 is coupled to the signal input of aconventional digital filter 82 which is operatively arranged to separatethe process generated noise component of the bath resistance variation(corresponds to a low frequency signal) from the longer term slowlyvarying component of bath resistance (corresponds substantially to DC);this latter component being determined principally by expected changesin the bath concentrations during normal operation. An output whichconstitutes the slowly varying component of bath resistance, from thedigital filter 82 is coupled to a first input of a first digital signalcomparator 44 and a first input of second digital signal comparator 45.A second input to the digital signal comparator 44 is provided from anupper threshold setting circuit 46 which is a source of a binary codeddigital signal for establishing an upper resistance value for the bath13, the alumina concentration being directly related to the resistanceof the bath 13. A second input to the second digital comparator 45 isprovided from a lower threshold setting circuit 47. The comparator 44provides an output whenever the digital signal it receives from thedigital filter 82 exceeds the digital signal it receives from the upperthreshold setting circuit 46, indicating that the resistance of the bathI3 is too high. It is to be understood that the digital signal source 42and the subtractor 43 are not necessary, the output from the divider 40could be directly coupled to the input of the digital filter 82 providedthat the threshold setting circuits were appropriately set to includethe fixed resis tance ofthe electrical connections to the reduction cell9.

The output from the limiting amplifier 35 is also connected to an anodeeffect detector 48 which is a Zener diode having a voltage switchingthreshold of approximately volts. Since the anode effect detector 48 hasa voltage threshold of 7.5 volts, it will not conduct and will notproduce an output signal so long as the voltage of the reduction cell 9remains within the range below 7.5 volts, the expected range being fromabout 3.5 volts to about 6.5 volts, 5.0 volts rarely being exceeded,during normal bath conditions. Whenever the voltage across the reductioncell 9 increases above the 7.5 volt level, the anode effect detector 48conducts producing a logical ONE signal on its output, indicating thatthe reduction cell 9 is undergoing an anode effect which signals thatthe concentration of alumina in the bath 13 is much too low forefficient operation. Since anode effect may and often does producevoltages as high as 30 or 40 volts across a reduction cell, the limitingamplifier 35 is arranged to limit at an input of about 10 volts therebypreventing damage to the'analog to digital converter 36 and to the anodeeffect detector 48 without decreasing'the sensitivity of the circuitry.

As discussed above, the circuitry as thus far described in effectdetermines the resistance of the reduction cell five times every minute.In practical applications of the present invention, the resistance ofthe re duction cell may be determined at greater intervals, for example,at one minute intervals.

A second output from the subtractor 43 is coupled to a conventionaldigital filter 83, which functions to pass the digital signalrepresentative of the process generated noise component of the bathresistance. An output from the digital filter 83 is coupled to a firstinput of a third digital signal comparator 84 which has its second inputcoupled to an output from a third threshold setting circuit 85. Thethreshold setting circuit 85 is set to provide as its output signal, adigital signal corresponding to that level of process generated noisewhich is to be tolerated in the apparatus. The comparator 84 provides anoutput signal whenever the digital signal it receives from the digitalfilter 83 is greater than that of the digital signal indicative of thelevel of noise to be tolerated, which it receives from the thresholdsetting circuit 85.

The output from the comparator 84 which, as a practical matter providesan output every minute, for example, is coupled to a three stage shiftregister 86 which is provided with a shift pulse input from the timingcircuit 32, which provides a shift pulse once every minute. Thus theoutput from the comparator 84 is effectively stored in the shiftregister 86 once every minute and shifted through the shift register 86in three steps, appearing as a binary ONE or ZERO at the output of itsfinal stage at the end of a three minute interval, de pending on thesignal received from the comparator 84. Likewise binary ONE or ZEROsignals appear in the first and second stages of the shift register.

In the event that the output signal from each of the three stages of theshift register 86 is a ONE, the AND circuit 87 responds, producing abinary ONE signal which is coupled, as an enabling signal, to thedigital signal source 88 and, as a reset signal to the shift register86. Thus, in order to obtain a binary ONE output from the AND circuit87, the noise signal must, in effect, have too great a magnitude for atleast three consecutive minutes.

Four digital sources 50, 51, 52, and 88 are provided. Each of thedigital signal sources 50, 51, 52, and 88 include respective stores 53,54, 55, and 89 which respectively store a regular normal break and feedprogram, a resistance control, anode position adjusting program, ananode effect extinguishing program, and a noise pot suppression program.The stored programs, in each instance, are respective stored binarycoded digital signals in bit parallel and command serial.

The digital signal source 50 provides in command sequence and bitparallel a series of binary coded digital command signals from its store53 to effect, in sequence, the breaking of the crust 16 on the tap sideby the breaker bar 24, the feeding of additional alumina to the tap sidefrom the feeder 24, the breaking of the crust 16 on the duct side by thebreaker bar 27 and the feeding of additional alumina to the duct sidefrom the feeder 26. The breaking bars 25 and 27 are, in most practicalinstances, moved up and down several times to assure that the crust 16is broken, the digital signal source 50 from its store 53 supplying theappropriate command signal or signals for effecting such multiplemotions.

In a practical instance, the digital signal source 50 supplies thedigital command signals, in bit parallel, which effects first a breakingat the tap side, with subsequent feeding of the tap side at apredetermined later time and thereafter, usually approximately 90minutes later, the breaking and subsequent feeding of the duct side ofthe reduction cell 9. Since the crust 16 is predominantly alumina, thebreaking of the crust 16 enriches the bath 13, resulting in a loweringof the bath resistance. The feeding may also provide, if desired,additional alumina to the bath 13, but is preferably done at a timesufficiently later than the breaking so that the newly fed aluminabecomes part of the crust 16 or is supported on its surface. The digitalsignals, in bit par allel, are supplied from the output of the digitalsignal source 50 to a command decoder 59 via a series connected negatedAND circuit 56, a negated AND circuit 57 and an OR circuit 58.

The digital signal source 51 is provided with two enabling inputs whichare supplied respectively from the comparator 44 and the comparator 45.ln response to a digital difference signal from the comparator 44,indicating that the upper threshold set point for the resistance of theelectrolytic reduction cell 9 has been exceeded, the digital signalsource 51 is operatively arranged to supply from its store 54 a binarycoded digital signal, in hit parallel, to the command decoder 59 callingfor the anode to be lowered by a given increment or increments dependingon the magnitude of the digital difference signal supplied from thecomparator 44. Thus, the resistance of the reduction cell 9 is lowereduntil the digital difference signal from the comparator 44 disappears.In response to a digital difference signal from the comparator 45,indicating that the lower threshold set point for the resistance of thereduction cell 9 has been exceeded, the digital signal source 51 isoperatively arranged to supply from its store 54 a binary coded digitalsignal, in bit parallel, to the command decoder 59 calling for the anode15 to be raised by a given increment or increments depending on themagnitude of the digital difference signal supplied from the comparator45. Consequently, the resistance of the reduction cell 9 is increaseduntil the digital difference signal from the comparator 45 disappears.

The binary coded digital signals from the digital signal source 51 whichcall for either an incremental lowering or an incremental raising of theanode 15, are supplied to the command decoder 59 via a negated ANDcircuit 60 and the OR circuit 58. A second output from the digitalsignal source 5], which simply indicates that the digital signal source51 is supplying signals to effect anode movement, is coupled to thenegated input of the AND circuit 57 thereby interrupting the regularbreak and feed program fed to the command decoder 59 from the digitalsignal source 50.

The digital signal source 5] as thus far described responds wheneverdifference signals appear on either the output from the comparator 44 orthe comparator 45. The digital signal source 51 is preferably soconstructed that it inhibits itself from supplying command signals for aperiod of five minutes after each of its responses.

The output from the anode effect detector 48, which appears as a logicalONE whenever its input exceeds 7.5 volts by virtue of the Zenercharacteristic of the detector 48, is coupled to the enabling input ofthe digital signal source 52. Whenever the digital signal source 52 isenabled, it produces from its store 54 a series of binary coded digitalcommand signals, in bit parallel, to effect in sequence the breaking ofthe crust 16 on both the tap side and the duct side of the reductioncell 9, the lowering of the anode l5, and the subsequent feeding of thereduction cell from both the feeder 24 and the feeder 26. As in thenormal breaking and feeding operation, the feeding operations preferablytake place during an anode effect extinguishing operation after thecrust 16 has hardened. In some instances, it may be sufiicient to breakand feed only either the duct side or the tap side to assure anodeeffect suppression.

The output digital command signals, in bit parallel, are coupled to thecommand decoder 59 via the OR circuit 58.

A second output from the digital signal source 52, which simplyindicates that the digital signal source 52 is providing an anode effectextinguishing command signal, is coupled to first negated inputs of theAND circuit 60 and of the AND circuit 56 and of an AND circuit for thepurpose of disabling feed of the regular break and feed program routinesignals, the regular, routine resistance adjusting anode positioningsignals and the noisy pot suppression signals to the command decoder 59.

Thus, the digital signal sources 50, 51, 52 and 88 supply to theexclusion of each other and on a priority basis coded digital commandsignals, in bit parallel, to the command decoder 59 which, in turn,produces 6 output signals on its output lines 61-66 which are fed torespective memory circuits 67-72. The memory circuits 67-72 in turnsupply signals to respective alternating current solenoid drivers 73-78.The memory circuits 67-72, which may be in the form of long RC timeconstant circuits, are provided to assure that the output of the commanddecoder 59 is present sufficiently long to energize their associatedrespective solenoid drivers 73-78, and at the same time freeing thecommand decoder 59 for the decoding of additional command signals.

The solenoid drivers 73 and 78, which respond respectively to signalsstored in the memory circuit 67 and in the memory circuit 72, arearranged to energize respectively the first feeder 24 on the tap sideand the second feeder 26 on the duct side of the electrolytic cell 9.The feeders 24 and 27 are of conventional construction and preferablyare operated by pneumatically or electrically responsive devicesrespectively controlled from the solenoid driver 73 and the solenoiddriver 78.

The solenoid drivers 74 and 77, which respond respectively to signalsstored in the memory circuit 68 and in the memory circuit 71, arearranged to energize respectively first and second pneumatically orelectrically operated devices 80 and 81 which are mechanically coupledrespective to the breaker bars 25 and 27 to effect movement of them.

The solenoid drivers 75 and 76, respond respectively to signals storedin the memory circuit 69 and the memory circuit 70, are arranged toenergize respectively the pneumatically or electrically operated motionproducing device 30 and the pneumatically or electrically operatedmotion-producing device 28 which are respectively operatively arrangedto effect the lowering and the raising of the anode 15.

A third output from the digital signal source 88 is fed to each of thethreshold setting circuits 46 and 47 for the purpose of adjustingupwardly the high and low thresholds, effectively changing upwardlly theresistant set point for the bath 13 upon initiation of a noise potsuppressing routine, and for returning these thresholds to theiroriginal set points after a time interval, preferably in increments.

In order to place the apparatus of the present invention in a conditionready for operation, suitable programs in the form of binary codeddigital signals for the regular, normal breaking and feeding function,for the resistance control function, for the anode effect extinguishingfunction, and for the noisy pot suppression are placed respectively inthe stores 53, 54, 55 and 89. Having determined, by conventionaltechniques, the subtantially fixed electrical resistance of theelectrical connections to the reduction cell 9, the digital signalsource 42 is set to provide, as its output signal, a binary codeddigital signal representative of such resistance. The digital signalsource 29 is set to provide, as its output signal, a binary codeddigital signal representative of the predetermined back EMF of thereduction cell 9, this back EMF being for a suitable alumina/cryolitebath 1.6 volts.

The upper threshold setting circuit 46 is set to provide, at its outputsignal, a fixed binary coded digital signal which corresponds to theupper limit (i.e., l X 10 ohms) of the resistance range for theelectrolytic bath 13 during expected normal electrolysis. This setpoint, for example, corresponds closely to that point at which the grossvoltage across the reduction cell 9 would have increased bysubstantially +0.02 volts at a nominal current of 150,000 amperes. Thelower threshold setting circuiit 47 is set to provide, as its outputsignal, a fixed binary coded digital signal which corresponds to thelower limit (i.e., l9.9X l0 ohms) of the resistance range for theelectrolytic bath 13 during expected normal electrolysis. This setpoint, for example, corresponds closely to that point at which the grossvoltage across the reduction cell 9 would havce decreased bysubstantially 0.02 volts at the nominal current of l50,000 amperes. Itis to be understood that different set points could be used if desired,as determined by desired bath conditions and the sensitivity of thecontrol desired in any given case. The threshold setting circuit is setto provide as its output signal, a fixed binary coded digital signalwhich corresponds to the level of process generated noise which is to betolerated.

The reduction cell 9 is charged with the appropriate amount of solvent,NaF/AIF and alumina, A1 0 which charge forms the electrolytic bath. Thereduction process is initiated preferably manually by supplying directcurrent to the reduction cell 9, with the possible addition of heat formauxiliary heating means, and adjusting manually the position of theanode 15, with respect to the cathode bottom of the reduction cell untilthe voltage across the reduction cell 9, as readable from the voltmeter31, and the direct current to the reduction cell 9, as determined by thecurrent sensing de vice 20, are within limits known to provide efficientoperation.

Once normal electrolysis is progressing, the digital signal source 50 isbrought into operation supplying regular break and feed digital commandsignals to the command decoder 59 which responds to such signals bysequentially signaling, via the memory circuits 68, 67, 71, and 72, thesolenoid drivers 74, 73, 77, and 78 which, in turn effect the movementof the breaking bar 25, the feeder 24, the breaking bar 27, and thefeeder 26. In normal operation, the tap side of the reduction cell 9 isthus broken and fed every 180 minutes, a delay period being providedbetween the breaking and feeding. The duct side of the electrodic cell 9is thus broken and fed also every 180 minutes, the times of each beingdisplaced by minutes from the corresponding breaking and feeding at thetap side of the reduction cell.

Electrolysis continues, the circuitry automatically determining theresistance of the bath l3, appropriate signals being produced by thecomparator 44 and the comparator 45 which, whenever the resistance ofthe bath 13 becomes either too high or too low, signal the digitalsignal source 51 which supplies digital command signals to the decoder59. The decoder 59 responds by producing, as the case may be, an outputsignal to either the memory circuit 69 or the memory circuit 70, whichcause the anode 15 to be either raised or low ered. This is accomplishedby the motion-producing devices 28 and 30 controlled from the solenoiddrivers 76 and 75, which respond to the signals stored in the memorycircuits 70 and 69 respectively. Whenever the digital signal source 51is supplying output signals, the output form the digital source 50 iseffectively prevented from reaching the command decoder 59 because ofthe fact that a signal from the digital signal source 51 is coupled tothe negated input of the AND circuit 57.

During the operation, the voltage across the reduction cell 9 isintermittently sensed, by action of the gate circuit 34, the voltagesignal being passed by the limiting amplifier 36, which has a gain ofone, its output in turn being supplied to the anode effect detector 48which conducts whenever the voltage exceeds 7.5 volts, its Zenerbreakdown voltage. The anode effect detector 48 responds within a fewmicroseconds, much faster than the 20 to 50 millisecond response time ofanalog to digital converter 36,supplying a logical ONE ignal to thedigital signal source 52 which produces a cries of digital commandsignals to the command decoder 59 to cause, in succession, the crust 16on the bath 13 to be broken, possibly on both the tap side and ie ductside of the reduction cell 9, the anode to re lowered, and subsequentfeeding of one or both ides of the reduction cell. The mechanicalmovements we effected by the solenoid drivers 75, 74, 73, 77 and "8. Thedigital signal source 52 also preferably pro =uces a digital commandsignal which is decoded by the lecoder 59 and fed to the solenoid driver76, via the nemory circuit 70, to cause the anode 15 to be re- 'urned toits earlier position.

A separate output from the digital signal source 52 is ed to negatedinputs of the AND circuits S6, 60, and 90 to assure that no commandsignals from the digital signal sources 50, 51,, and 88 are supplied tothe command decoder 59 when it is receiving command signals from thedigital signal source 52.

During the operation of the apparatus, as so far described, the digitalfilter 83 passes digital signals representing the noise component ofbath resistance to the :omparator 84 which provides a binary ONE outputsignal upon the signal passed by the digital filter 83 exseeding theoutput from the threshold setting circuit 35. The output from thecomparator 84 is stored in the nput stage of the three-stage shiftregister 86 and shifted, at one minute intervals, through the shiftregister 86 stage by stage. Since an output from each of the threestages of the shift register 86 are connected to separate inputs of theAND circuit 87, the AND circuit 87 produces a binary ONE output signalonly when ONE signals are present at the output of each of the threestages, a condition which prevails only when the comparator 84 hassignaled the presence of too high a process generated noise level. Thebinary ONE output signal form the AND circuit 87 is fed as a resetsignal to the shift register 86, thus effecting the resetting of theshift register 87 whenever the noise level has exceeded the thresholdlevel for a three minute interval.

The binary ONE signal from the AND circuit 87 is fed to the digitalsignal source 88 which supplies a digital command signal from its store89 to the decoder 59, via the AND circuit 90 and the OR circuit 58. Thedigital command signal is decoded by the decoder 59 and fed via thememory circuit 70 to the solenoid driver 76 which raises the anode 15 apredetermined amount known to be sufficient to reduce the magnitude ofnoise component of resistance a given amount in most cases. Of course,if the shift register 86 still indicates that the noise level remainstoo high, further command signals are provided by the dig tal signalsource 88 causing the anode 15 to be raised further until the noiselevel is within an acceptable liznit.

So long as the digital source 88 is producing command signals forraising the anode 15, digital signals from the digital signal sources 50and 51 are blocked from the OR circuit 58 because of negated inputs ofthe AND circuits 56 and 60 receiving a binary ONE signal from thedigital source 88. It is to be appreciated, however, that if the digitalsignal source 52 were to produce command signals, they would pass to thedecoder 59 and the command signals from the digital source 88 would beblocked by virtue of the negated input to the AND circuit 90 from thesecond output of the digital signal source 52.

A further output signal from the digital signal source 88 is fed to thethreshold setting circuits 46 and 47 to raise the upper and lowerthresholds, effectively setting a higher set point for the resistance ofthe bath 13.

Since it is expected that the cause of a noisy pot will in time becorrected or that the movement of the anode 15 itself may affectsuppression of the cause of an anode effect, the digital signal source88 is operatively arranged to supply to the decoder 59 from its store 89a command signal or signals which are supplied to the motion-producingdevice 30 via the solenoid driver and the memory circuit 69 which causethe anode 15 to be lowered to its initial position or toward its initialposition in increments, the anode movement ceasing whenever it becomespositioned in its initial position. Of course, the set points of thethreshold circuits 46 and 47 are again set to or toward their initiallevels, in accordance with command signals reflecting the changingposition of the anode 15.

In the event the shift register 86 again signals the occurrence of toohigh a noise level for too long a time, three minutes, during the returnof the anode 15 toward its initial position, the noisy pot suppressionroutine is again activated. This will continue time and again untileither the noisy pot phenomena is effectively suppressed or an operator,knowing the phenom ena persists, sets the threshold circuits 46 and 47to higher values thereby changing the effective set point of theresistance for the bath 13 to a new higher value at which too high alevel of process generated noise does not occur.

The method of producing metal according to the present invention, it itsbroadest aspect, involves the steps of providing an electrolytic bathcontaining dissolved oxide of the metal in a reduction cell, causingdirect current of flow through the oath, collecting the metal producedon the bottom of the reduction cell, sensing the process generated noisecomponent of resistance of the bath and determining when the componentexceeds a given level as an indication of an undesirable noise level.

[n a further aspect, the method according to the present inventionincludes the step of reducing the noise level whenever it exceeds thegiven level and, more particularly, when the given level is exceeded fora given period of time, for example, 3 minutes.

The method according to the present invention preferably includesreducing the noise component by increasing the spacing between anode andcathode electrodes associated with the bath.

The method according to a preferred aspect, involves the production ofaluminum. in this cas the electrolytic bath is composed of alumina asthe solute and cryolite as the solvent.

The method according to a preferred aspect includes the step of reducingthe spacing between the cathode and anode after a period subsequent tothe increasing step. This is done preferably in discrete increments.

In another preferred aspect, the method according to the presentinvention involves the reestablishment of the original spaceing betweenanode and cathode.

Although the present invention has been described, in its apparatusaspect, in conjunction with a single electrolytic reduction cell, it isto be appreciated that the invention is applicable to systems whichinvolve multiplexing of the command signals in order to control theoperating parameters of many reduction cells. in

this instance, the circuit arrangement would, of course, also sense thecurrents supplied to each cell, the voltages across each cell viamultiplexing circuits, the feeding of the command signals and thesensing of the cur rents and the voltages being appropriatelysynchronized.

The term digital filter, as is readily understood by those skilled inthe art, in the fields of numerical analyses and computers refers to anytechnique of digital cir cuit which smooths or selectively passes data,while rejecting other data. It is to be appreciated that the digitalfilters 82 and 83 are conventional and may correspond to conventionanalog signal filters having resistance, capacitances and/orinductances. It is to be appreciated that the digital filter 83 may be aleast squares estimator.

[t is also to be appreciated that the invention, in its method aspect,need not be carried out in the illustrated apparatus, but may be carriedout by other apparatuses.

While one embodiment of the invention has been shown for purposes ofillustration, it is to be understood that various changes in the detailsof construction and arrangement of parts may be made without departingfrom the spirit and scope of the invention as defined in the method andapparatus claims.

It is claimed:

1. A mehtod of producing metal comprising providing an electrolytic bathcontaining dissolved oxide of the metal in a reduction cell, causingdirect current to flow through said bath, collecting said metal on thebottom of said reduction cell, electrically sensing the processgenerated noise component of resistance of said bath and electricallydetermining when such component exceeds a given level as an indicationof an undesirable noise level.

2. A method as defined in claim 1 further comprising reducing saidprocess generated noise component of resistance at least to said givenlevel upon determining that said noise component exceeds said givenlevel.

3. A method as defined in claim 2 wherein said step of reducing saidnoise component of resistance is effected by increasing the spacingbetween anode and cathode electrodes associated with said bath.

4. A method as defined in claim 3 further including reducing the spacingbetween said anode and cathode after a predetermined period subsequentto the step of increasing the spacing.

5. A method as defined in claim 4 wherein the step of reducing thespacing is accomplished in a series of incremental steps.

6. A method as defined in claim 4 wherein said reducing step effects areestablishing of substantially the original anode-to-cathode spacing.

7. A method as defined in claim 1 wherein the step of providing anelectrolytic bath comprises providing an electrolytic bath composed ofalumina as the solute and cryolite as the solvent, the metal producedbeing aluminum.

8. A mehtod as defined inclaim 1 wherein the step of electricallydetermining when said component exceeds a given level determines if saidgiven level has been exceeded for a given period of time.

9. An apparatus for producing metal from an electrolytic bath containingdissolved oxide of the metal comprising at least on reduction cellhaving electrode means for delivering direct current to the electrolyticbath, said electrode means including anode electrode means and cathodeelectrode means; means for sensing the process generated noise componentof the resistance of said bath and means responsive to output from saidmeans for sensing for determining the occurrence of noise components inexcess of a given level.

10. An apparatus as defined in claim 9 further comprising meansresponsive to output from said means for determining for reducing thenoise component at least to the given level.

11. An apparatus as defined in claim 10 wherein said means for reducingthe noise component comprise means for increasing spacing between saidanode electrode means and said cathode electrode means.

12. An apparatus as defined in claim 11 futher comprising meansoperative subsequently to said means for increasing spacing for reducingthe spacing between said anode electrode means and said cathodeelectrode means.

13. An apparatus as defined in cliam 12 wherein said means for reducingthe spacing between said anode electrode means and said cathodeelectrode means is operatively arranged to reestablish the spacingbetween said anode electrode means and said cathode electrode meanssubstantially to the original spacing.

14. An apparatus as defined in claim 12 wherein said means for reducingthe spacing is a means for reducing the spacing in incrementaldistances.

15. An apparatus as defined in claim 13 wherein said means for reducingthe spacing is a means for reducing the spacing by incremental distancestoward the original spacing.

16. An apparatus as defined in claim 9 further comprising means fordetermining the resistance of said bath and means responsive to outputfrom said means for determining resistance for adjusting spacing betweensaid anode electrode means and said cathode electrode means to maintainthe resistance of said bath within predetermined limits.

17. An apparatus as difined in claim 9 wherein said means fordetermining the occurrence of noise components in excess of a givenlevel are means for determining the substantially continuous occurrenceof noise components in excess of said given level for a given period oftime.

18. In an apparatus for producing metal from an electrolytic bathcontaining dissolved oxide of the metal which includes at least onereduction cell having electrode means for delivering direct current tothe electrolytic bath, said electrode means including anode electrodemeans and cathode electrode means, the improvement which comprises:

means for producing a resistance signal indicative of the resistance ofsaid electrolytic bath, said resistance signal having a processgenerated noise component and a normal bath concentration component;

means responsive to said resistance signal for separating said processgenerated noise component and said normal bath concentration componentinto first and second signals, respectively;

means responsive to said first signal for incresing the spacing betweensaid anode electrode and said cathode electrode if said first signalexceeds a predetermined maximum tolerable value for said processgenerated noise component whereby the latter resistance component ofsaid electrolytic bath will be reduced to at least said tolerable value;and

means responsive to said second signal for respectively increasing ordecreasing the spacing between said anode electrode and said cathodeelectrode if said second signal falls below or above predeter' minedminimum or maximum values, respectively, for said normal bathconcentration resistance component.

19. The apparatus according to claim 18, further comprising means forinhibiting the operation of said means responsive to said second signalin response to the activation of said means responsive to said firstsignal.

20. The apparatus according to claim 19, further comprising means forproducing a voltage signal indicative of the voltage across saidelectrode means, said means for producing a resistance signal beingresponsive to said voltage signal, and means further responsive to saidvoltage signal when said voltage signal exceeds a predetermined maximumlevel for activating means for feeding dissolved oxide to saidelectrolytic bath whereby said voltage across said electrode means isreduced.

21. The apparatus according to claim 20, further comprising means forinhibiting the operation of said means responsive to said first signaland said means responsive to said second signal in response to theactivation of said means further responsive to said voltage signal.

22. The apparatus according to claim 19, wherein said means responsiveto said first signal comprises first threshold setting means for storingsaid predetermined maximum tolerable value for said process generatednoise resistance component; first comparator means for issuing an outputsignal when said first signal exceeds said maximum tolerabe value; shiftregister means responsive to said output signal for generating a binarysignal, said shift register means having a plurality of stages throughwhich said binary signal is shifted after a periodic time interval; andgating means responsive to the concomitant presence of a binary signalin each of said plurality of stages of said shift register for issuing acommand signal, and a digital signal source means responsive to saidcommand signal for effecting said increase in spacing between saidelectrodes.

23. The apparatus according to claim 22, wherein said means responsiveto said second signal comprises second threshold setting means forstoring said predetermined maximum and minimum values of said normalbath concentration resistance component, and second comparator means forissuing an outut signal when said second signal either falls above orbelow said maximum or minimum predetermined values.

24. The apparatus according to claim 23, wherein said second thresholdsetting means are responsive to an output from said digital signalsource means for resetting said maximum and minimum values of saidnormal bath concentration resistance component.

1. A METHOD OF PRODUCING METAL COMPRISING PROVIDING AN ELECTROLYTIC BATHCONTAINING DISSOLVED OXIDE OF THE METAL IN A REDUCTION CELL, CAUSINGDIRECT CURRENT TO FLOW THROUGH SAID BATH, COLLECTING SAID METAL ON THEBOTTOM OF SAID REDUCTION CELL, ELECTRICALY SENSING THE PROCESS GENERATEDNOISE COMPONENT OF RESISTANCE OF SAID BATH AND ELECTRICALLY DETERMININGWHEN SUCH COMPONENT EXCEEDS A GIVEN LEVEL AS AN INDICATION OF ANUNDERSIRABLE NOISE LEVEL.
 2. A method as defined in claim 1 furthercomprising reducing said process generated noise component of resistanceat least to said given level upon determining that said noise componentexceeds said given level.
 3. A method as defined in claim 2 wherein saidstep of reducing said noise component of resistance is effected byincreasing the spacing between anode and cathode electrodes associatedwith said bath.
 4. A method as defined in claim 3 further includingreducing the spacing between said anode and cathode after apredetermined period subsequent to the step of increasing the spacing.5. A method as defined in claim 4 wherein the step of reducing thespacing is accomplished in a series of incremental steps.
 6. A method asdefined in claim 4 wherein said reducing step effects a reestablishingof substantially the original anode-to-cathode spacing.
 7. A method asdefined in claim 1 wherein the step of providing an electrolytic bathcomprises providing an electrolytic bath composed of alumina as thesolute and cryolite as the solvent, the metal produced being aluminum.8. A mehtod as defined inclaim 1 wherein the step of electricallydetermining when said component exceeds a given level determines if saidgiven level has been exceeded for a given period of time.
 9. Anapparatus for producing metal from an electrolytic bath containingdissolved oxide of the metal comprising at least on reduction cellhaving electrode means for delivering direct current to the electrolyticbath, said electrode means including anode electrode means and cathodeelectrode means; means for sensing the process generated noise componentof the resistance of said bath and means responsive to output from saidmeans for sensing for determining the occurrence of noise components inexcess of a given level.
 10. An apparatus as defined in claim 9 furthercomprising means responsive to output from said means for determiningfor reducing the noise component at least to the given level.
 11. Anapparatus as defined in claim 10 wherein said means for reducing thenoise component comprise means for increasing spacing between said anodeelectrode means and said cathode electrode means.
 12. An apparatus asdefined in claim 11 futher comprising means operative subsequently tosaid means for increasing spacing for reducing the spacing between saidanode electrode means and said cathode electrode means.
 13. An apparatusas defined in cliam 12 wherein said means for reducing the spacingbetween said anode electrode means and said cathode electrode means isoperatively arranged to reestablish the spacing between said anodeelectrode means and said cathode electrode means substantially to theoriginal spacing.
 14. An apparatus as defined in claim 12 wherein saidmeans for reducing the spacing is a means for reducing the spacing inincremental distances.
 15. An apparatus as defined in claim 13 whereinsaid means for reducing the spacing is a means for reducing the spacingby incremental disTances toward the original spacing.
 16. An apparatusas defined in claim 9 further comprising means for determining theresistance of said bath and means responsive to output from said meansfor determining resistance for adjusting spacing between said anodeelectrode means and said cathode electrode means to maintain theresistance of said bath within predetermined limits.
 17. An apparatus asdifined in claim 9 wherein said means for determining the occurrence ofnoise components in excess of a given level are means for determiningthe substantially continuous occurrence of noise components in excess ofsaid given level for a given period of time.
 18. In an apparatus forproducing metal from an electrolytic bath containing dissolved oxide ofthe metal which includes at least one reduction cell having electrodemeans for delivering direct current to the electrolytic bath, saidelectrode means including anode electrode means and cathode electrodemeans, the improvement which comprises: means for producing a resistancesignal indicative of the resistance of said electrolytic bath, saidresistance signal having a process generated noise component and anormal bath concentration component; means responsive to said resistancesignal for separating said process generated noise component and saidnormal bath concentration component into first and second signals,respectively; means responsive to said first signal for incresing thespacing between said anode electrode and said cathode electrode if saidfirst signal exceeds a predetermined maximum tolerable value for saidprocess generated noise component whereby the latter resistancecomponent of said electrolytic bath will be reduced to at least saidtolerable value; and means responsive to said second signal forrespectively increasing or decreasing the spacing between said anodeelectrode and said cathode electrode if said second signal falls belowor above predetermined minimum or maximum values, respectively, for saidnormal bath concentration resistance component.
 19. The apparatusaccording to claim 18, further comprising means for inhibiting theoperation of said means responsive to said second signal in response tothe activation of said means responsive to said first signal.
 20. Theapparatus according to claim 19, further comprising means for producinga voltage signal indicative of the voltage across said electrode means,said means for producing a resistance signal being responsive to saidvoltage signal, and means further responsive to said voltage signal whensaid voltage signal exceeds a predetermined maximum level for activatingmeans for feeding dissolved oxide to said electrolytic bath whereby saidvoltage across said electrode means is reduced.
 21. The apparatusaccording to claim 20, further comprising means for inhibiting theoperation of said means responsive to said first signal and said meansresponsive to said second signal in response to the activation of saidmeans further responsive to said voltage signal.
 22. The apparatusaccording to claim 19, wherein said means responsive to said firstsignal comprises first threshold setting means for storing saidpredetermined maximum tolerable value for said process generated noiseresistance component; first comparator means for issuing an outputsignal when said first signal exceeds said maximum tolerabe value; shiftregister means responsive to said output signal for generating a binarysignal, said shift register means having a plurality of stages throughwhich said binary signal is shifted after a periodic time interval; andgating means responsive to the concomitant presence of a binary signalin each of said plurality of stages of said shift register for issuing acommand signal, and a digital signal source means responsive to saidcommand signal for effecting said increase in spacing between saidelectrodes.
 23. The apparatus according to claim 22, wherein said meansresponsive to said second signal comprises secOnd threshold settingmeans for storing said predetermined maximum and minimum values of saidnormal bath concentration resistance component, and second comparatormeans for issuing an outut signal when said second signal either fallsabove or below said maximum or minimum predetermined values.
 24. Theapparatus according to claim 23, wherein said second threshold settingmeans are responsive to an output from said digital signal source meansfor resetting said maximum and minimum values of said normal bathconcentration resistance component.